The present invention relates to a variable displacement compressor used in vehicle air conditioners. Specifically, the present invention pertains to a device and a method for controlling the displacement of a variable displacement compressor.
FIG. 14 shows a prior art variable displacement compressor. The compressor includes a housing 101. A crank chamber 102 is defined in the housing 101. A drive shaft 103 is rotatably supported in the housing 101. A lip seal 104 is located between the housing 101 and the drive shaft 103 to prevent gas leakage along the surface of the drive shaft 103.
The drive shaft 103 is connected to a vehicle engine Eg, which serves as an external power source, through an electromagnetic friction clutch 105. The friction clutch 105 includes a pulley 106, an armature 107 and an electromagnetic coil 108. When the clutch 105 engages, that is, when the coil 108 is excited, the armature 107 is attracted to and is pressed against the pulley 106. As a result, the clutch 105 transmits the driving force of the engine Eg to the drive shaft 103.
When the clutch 105 disengages, that is, when the coil 108 is de-excited, the armature 107 is separated from the pulley 106. In this state, the driving force of the engine Eg is not transmitted to the drive shaft 103.
A rotor 109 is secured to the drive shaft 103 in the crank chamber 102. A thrust bearing 122 is located between the rotor 109 and the inner wall of the housing 101. A swash plate 110 is coupled to the rotor 109 by a hinge mechanism 111. The hinge mechanism 111 permits the swash plate 110 to rotate integrally with the drive shaft 103 and to incline with respect to the axis L of the drive shaft 103. When the swash plate 110 abuts against a limit ring 112 fitted about the drive shaft 103 as illustrated by two-dot chain line in FIG. 14, the swash plate 110 is at the minimum inclination position. When the swash plate 110 abuts against the rotor 109 as illustrated by solid line in FIG. 14, the swash plate 110 is at the maximum inclination position.
Cylinder bores 113, suction chamber 114 and a discharge chamber 115 are defined in the housing 101. A piston 116 is reciprocally housed in each cylinder bore 113. The pistons 116 are coupled to the swash plate 110. The housing 101 includes a valve plate 117. The valve plate 117 separates the cylinder bores 113 from the suction chamber 114 and the discharge chamber 115.
Rotation of the drive shaft 103 is converted into reciprocation of each piston 116 by the rotor 109, the hinge mechanism 111 and the swash plate 110. Reciprocation of each piston 116 draws refrigerant gas from the suction chamber 114 to the corresponding cylinder bore 113 via a suction port 117a and a suction valve flap 117b, which are formed in the valve plate 117. Refrigerant gas in the cylinder bore 113 is compressed to reach a predetermined pressure and is discharged to the discharge chamber 115 via a discharge port 117c and a discharge valve flap 117d, which are formed in the valve plate 117.
A spring 118 urges the drive shaft 103 forward (to the left as viewed in FIG. 14) along the axis L through a thrust bearing 123. The spring 118 prevents axial chattering of the drive shaft 103.
The crank chamber 102 is connected to the suction chamber 114 by a bleeding passage 119. The discharge chamber 115 is connected to the crank chamber 102 by a supply passage 120. The opening of the supply passage 120 is regulated by an electromagnetic displacement control valve 121.
The control valve 121 adjusts the opening of the supply passage 120 thereby regulating the amount of pressurized refrigerant gas drawn into the crank chamber 102 from the discharge chamber 115. The pressure in the crank chamber 102 is changed, accordingly. As a result, the inclination of the swash plate 110 is altered and the stroke of each piston 116 is changed, which varies the compressor displacement.
When the clutch 105 disengages or when the engine Eg is stops, the control valve 121 fully opens the supply passage 120. This increases the pressure in the crank chamber 102 and decreases the inclination of the swash plate 110. The compressor stops operating with the swash plate 110 at the minimum inclination position. When the compressor is started again, the displacement of the compressor is minimum, which requires minimum torque. The shock caused by starting the compressor is thus reduced.
When there is a relatively great cooling demand on a refrigeration circuit that includes the compressor of FIG. 14, for example, when the temperature in a passenger compartment of a vehicle is much higher than a target temperature set in advance, the control valve 121 closes the supply passage 120 and maximizes the compressor displacement.
When the clutch 105 disengages or when the engine Eg is stopped, the compressor is stopped. If the compressor is stopped when operating at the maximum displacement, the control valve 121 quickly and fully opens the closed supply passage 120. Also, when the vehicle is suddenly accelerated while the compressor is operating at the maximum displacement, the control valve 121 quickly and fully opens the supply passage 120 to minimize the displacement to reduce the load applied to the engine.
Accordingly, highly pressurized refrigerant gas in the discharge chamber 115 is quickly supplied to the crank chamber 102, which rapidly increases the pressure in the crank chamber 102. Refrigerant gas in the crank chamber 102 constantly flows to the suction chamber 114 through the bleeding passage 119. However, since the amount of refrigerant gas that flows to the suction chamber 114 through the bleeding passage 119 is limited, the pressure in the crank chamber 102 is quickly increased an excessive level.
The sudden increase of the crank chamber pressure suddenly moves the swash plate 110 from the maximum inclination position to the minimum inclination position, which causes the swash plate 110 violently collides with the limit ring 112. The collision produces unpleasant noise. The swash plate 110 also strongly pulls the drive shaft 103 rearward (to the right as viewed in FIG. 14) through the ring 112 or through the hinge mechanism 111 and the rotor 109. As a result, the drive shaft 103 moves rearward along the axis L against the force of the spring 118.
When the drive shaft 103 moves rearward, the axial position of the drive shaft 103 relative to the lip seal 104, which is retained in the housing 101, changes. Normally, a predetermined annular area of the drive shaft 103 contacts the lip seal 104. Foreign particles and sludge adhere to a surface of the drive shaft 103 that is axially adjacent to the predetermined annular area. Therefore, if the axial position of the drive shaft 103 relative to the lip seal 104 changes, sludge enters between the lip seal 104 and the drive shaft 103. This lowers the effectiveness of the lip seal 104 and results in gas leakage from the crank chamber 102.
Particularly, when the drive shaft 103 moves rearward due to disengagement of the clutch 105, the armature 107, which is fixed to the drive shaft 103, moves toward the pulley 106. The clearance between the pulley 106 and the armature 107 is as small as 0.5 mm when the clutch 105 disengages. Rearward movement of the drive shaft 103 eliminates the clearance between the pulley 106 and the armature 107, which may cause the armature 107 to contact the rotating pulley 106. As a result, noise and vibration are produced. Also, even if the clutch 105 disengages, the driving force of the engine Eg is transmitted to the drive shaft 103.
When the drive shaft 103 moves rearward, the average position of the pistons 116, which are coupled to the drive shaft 103 by the swash plate 110, is moved rearward. This causes the top dead center of each piston 116 to approach the valve plate 117. If the compressor is operating, the pistons 116 may repeatedly collide with the valve plate 117, which produces vibration and noise.
To prevent the drive shaft 103 from moving rearward, the force of the spring 118 may be set greater. However, a greater force of the spring 118 increases load acting on the thrust bearings 122, 123 and increases power loss of the compressor.
If the compressor starts operating by engagement of the clutch 105 when there is a relatively great cooling demand on a refrigeration circuit that includes the compressor of FIG. 14, the control valve 121 suddenly closes the fully opened supply passage 120 to maximize the compressor displacement. Accordingly, the swash plate 110 moves from the minimum inclination position to the maximum inclination position and violently collides with the rotor 109. The collision produces unpleasant noise.
Japanese Unexamined Patent Publication No. 8-338364 also discloses a variable displacement compressor that has similar drawbacks as the compressor of FIG. 14.